The present invention relates generally to the abatement of contaminant laden industrial process emissions and more particularly, to a system which utilizes a regenerative catalytic oxidizer and a selective catalytic reduction unit to perform the abatement process.
Industrial process emissions typically contain particulates and major gaseous air pollutants such as volatile organic compounds (VOCs), nitrogen oxides (NO.sub.x) and carbon monoxide (CO), all of which contaminate the environment when vented to the atmosphere. Thermal oxidizers utilize a supplementary heat source to increase the temperature of the process emissions to a level above the ignition temperature of the combustible contaminants, typically ranging from 1400.degree. F. (.apprxeq.760.degree. C.) to 1500.degree. F. (.apprxeq.815.degree. C.), so as to oxidize combustible contaminants, such as VOC's and CO's. Catalytic oxidizers further utilize a catalytic material to effect oxidation of the VOCs and CO at lower peak temperatures.
Regenerative thermal and/or catalytic oxidizers recover heat remaining in the cleansed exhaust gas to increase the temperature of emissions entering the oxidizer thereby minimizing the amount of supplemental energy required to raise the emission to its ignition temperature. Characteristically, flow control valves are used to direct the emissions to one or more regenerators for preheating prior to thermal or catalytic oxidation.
Although known regenerative oxidizers effectively convert VOCs and CO to benign gases such as carbon dioxide (CO.sub.2) and water vapor, oxidation does not, however, reduce the level of toxic NO.sub.x from the process emissions. In contradistinction, attenuation of the NO.sub.x concentration chemically requires a reduction reaction, as opposed to oxidation in a regenerative thermal and/or catalytic oxidizer.
Known abatement methods therefore require a separate unit to remove NO.sub.x from the process emissions. In gas turbine technology, selective catalytic reduction units utilize a reducing gas to react with NO.sub.x at elevated temperatures over a selective catalyst, thereby forming harmless gases such as nitrogen (N.sub.2) and water vapor. Common examples of reducing gases include hydrogen, urea, ammonia, and cracked ammonia. The reduction reaction frequently occurs at temperatures ranging from 500.degree. F. (.apprxeq.260.degree. C.) to 700.degree. F. (.apprxeq.371.degree. C.) in the presence of a selective catalyst, or above 1200.degree. F. (.apprxeq.649.degree. C.) in the absence of a catalyst.
Alternatively, NO.sub.x may be removed by a scrubber. Generally, scrubbing is a process for removing one or more components of a mixture of gases or vapors. The gas mixture is passed upward and countercurrent to a stream of descending liquid, such as water, which removes the NO.sub.x contaminants by dissolving desired components and not others.
This bifurcated approach of removing VOCs and CO in a regenerative oxidizer and NO.sub.x in a separate unit has not proven to be satisfactory. Utilizing two separate pieces of equipment is expensive and inefficient. For example, the oxidizer effluent must be reheated in the catalytic reduction unit when reacting NO.sub.x with a reducing gas. Removing NO.sub.x in a scrubber is also problematic because waste liquid is generated, which creates yet another pollution source. Maintenance and operation of the two separate units is also expensive and cumbersome.